Generally either 2x2 or 2x3, because it's difficult to reach past the middle of anything wider. In MegaMek, I've played 3x3, but generally stick with 2x2 or 2x3 because it takes too long otherwise before the slower stuff comes into range on a map with woods or intervening hills.
In MegaMek, I've spent 30+ turns trying to find the last enemy unit, which turns out to be a hovertank surrounded by woods, a 'Mech in a water hex it can't climb out of, or something parked in some hidden alcove in a line of hills. Adding more mapsheets makes it worse.